1. Field of the Invention
The present invention relates to a pneumatic radial tire, particularly to a pneumatic radial tire for passenger cars, having a decreased weight and exhibiting excellent durability, riding comfort and controllability.
2. Description of the Related Art
Conventionally, steel cords have been used for carcasses of pneumatic radial tires for trucks and busses.
On the other hand, in general, organic fibers have been used for the carcass of pneumatic radial tires for passenger cars. As the organic fiber, polyesters are mainly used and rayon and nylon are occasionally used.
The reason why organic fibers have been used for the carcass of pneumatic radial tires for passenger cars can be considered to be as follows.
When a tire for passenger cars and a tire for trucks and busses are compared, reinforcement of the tire for passenger cars with a belt is smaller and the tire for passenger cars is used at a lower inner pressure. Greater compression stress works on the carcass of the tire for passenger cars because passenger cars undergo greater movement during turns. Therefore, when a tire having a steel cord in place of an organic fiber is prepared with the tire designed in a manner such that the strength of the carcass is kept the same, the steel cord having greater rigidity is fractured by the greater compression stress described above. For the above reason, organic fibers having less rigidity than that of steel cords have been used.
Recently, decreasing the weight of vehicles and saving fuel are required from the standpoint of energy saving and environmental protection. As for tires, the weight and rolling resistance are being decreased. From this standpoint, there have been attempts to convert the reinforcing material for the carcass of pneumatic radial tires from organic fibers to steel cords, to decrease the amount of the cord while maintaining the strength of the tire by increasing the strength of the carcass.
However, as described above, when a steel cord is used for a carcass of a pneumatic radial tire for passenger cars, the steel cord is subjected to repeated compression stress during rolling of the tire and buckling deformation of the steel cord takes place when the vehicle turns. Therefore, durability of the carcass deteriorates.
The present invention has been made under the above circumstances and an object of the present invention is to provide a pneumatic radial tire for passenger cars which can suppress deterioration in durability of a carcass which takes place due to buckling deformation when a steel cord is applied to the carcass of the pneumatic radial tire, which has a decreased weight due to application of a high strength steel cord and which exhibits improved riding comfort and controllability.
In a first aspect of the present invention, a pneumatic radial tire comprising a pair of bead portions, and a carcass as a skeletal component of the tire, constituted of a ply of radially disposed cords and extending in a toroidal form between the pair of bead portions, wherein the carcass including circumferentied steel cords, each steel cord having at least one steel filament, with a tensile rigidity, EL, the EL satisfying an equation (1) as follows:
1300xe2x89xa6EL(kgf/mm2)xe2x89xa62400
EL=ECxc3x97FA/(Dxc3x97S)xe2x80x83xe2x80x83(1)
wherein
EC is the Young""s modulus of the steel cord (kgf/mm2);
FA is the total sectional area of the filament forming the steel cord (mm2);
D is the diameter of the steel cord (mm); and
S is the pitch between the steel cords that are adjacent in a crown portion (mm).
In accordance with the first aspect of the present invention, tensile rigidity of the carcass is kept smaller than that of conventional carcasses by application of the steel cord having a smaller Young""s modulus to the carcass. Thus, buckling deformation of the steel cord is prevented and durability is improved.
In cases where the tensile rigidity of the carcass is kept smaller than that of conventional carcasses using steel cords, when the tire is inflated by an inner pressure tensile stress toward an outer side in the transverse direction of the tire works also on the carcass, and extension of the carcass becomes greater than that of conventional carcasses having higher tensile rigidity. As the result, tension toward the outer side in the transverse direction of the tire occurs also on the belt disposed in the direction perpendicular to the direction of the carcass cord. Thus, compressing input force toward the inner side in the transverse direction of the tire is alleviated even when such compressing input force is applied at the belt when the vehicle turns, and extension of the belt in the circumferential direction of the tire is suppressed. In other words, when the vehicle turns, compression stress is not generated at the steel cords in the carcass at the inner side of the equatorial plane of the tire (with respect to the turn in the tire) attached at the outer side of the vehicle with respect to the turn and durability of the carcass is improved.
In a second aspect of the present invention, tensile strength, TS, of the steel filament in the steel cord satisfies an equation (2) as follows:
TS(kgf/mm2)xe2x89xa6230xe2x88x92148 log dxe2x80x83xe2x80x83(2)
wherein
d is diameter of the steel filament (mm); and
log is common logarithm.
In accordance with the second aspect of the present invention, a high strength steel cord formed with steel filaments having a great tensile strength is used for the carcass. As a result, input force (tensile stress) per unit sectional area of the steel filament forming the steel cord increases when tensile force works on the carcass and the extension (elongation) increases. In other words, tensile rigidity of the carcass decreases. As the result, durability is improved due to the decrease in tensile rigidity of the carcass.
The amount of the cord necessary for maintaining the required strength of the carcass is decreased by the application of the high strength steel cord and the weight of the tire can be decreased.
In a third aspect of the present invention, the diameter of the steel filament in the steel cord is 0.10 to 0.32 mm.
When the diameter of the steel filament is smaller than 0.10 mm, bending rigidity of the carcass decreases to decrease rigidity of a side portion of the pneumatic radial tire excessively and controllability deteriorates. Although increasing tensile strength of the steel filament is advantageous in terms of decreasing the weight of the pneumatic radial tire, manufacturing productivity in the process of stretching the filament decreases, which is economically disadvantageous. When the diameter of the steel filament exceeds 0.32 mm, bending rigidity of the carcass increases excessively and riding comfort deteriorates.
Therefore, when the diameter of the steel filament forming the steel cords is 0.10 to 0.32 mm, rigidity of the carcass can be decreased to a suitable degree to improve durability, and excellent controllability and riding comfort can also be achieved.